Electric resistance welded steel pipes or steel tubes have been heretofore frequently used in the extraction or transportation of resources such as crude oil and natural gas. Electric resistance welded steel pipes or steel tubes have electric resistance welded parts. From the viewpoint of the reliability of electric resistance welded parts, the use of the steel pipes or tubes has been limited to applications which require low levels of hydrogen induced cracking resistance (hereinafter, also written as HIC resistance) and low-temperature toughness of electric resistance welded parts. Thus, there have been strong demands for electric resistance welded steel pipes or steel tubes having excellent HIC resistance and excellent low-temperature toughness.
To meet such demands, for example, Patent Literature 1 describes electric resistance welded steel pipes or steel tubes with excellent sour resistance that are made of Al deoxidized steel containing 0.0012% or more of calcium. According to the technique described in Patent Literature 1, the Ca/Al ratio is controlled to 0.10 or less, the ratio of the length in the through-thickness direction to the length in the circumferential direction is controlled to 2 or more, and the density of inclusions having a major diameter of 10 μm or more is controlled to 5 particles or less per 1 mm2. These configurations allegedly make it possible to prevent the occurrence of hydrogen induced blister cracking even under a severe environment with a low pH and enable the electric resistance welded steel pipes or steel tubes to exhibit excellent sour resistance.
Patent Literature 2 describes a method for the gas shield welding of electric resistance welded steel pipes or steel tubes which may enhance the toughness of electric resistance welded parts. The technique described in Patent Literature 2 is characterized in that suspended scales on the inner side of the pipe are removed by mist washing between after fin pass forming and before welding, and the local gas shielding of a welded part is performed in such a manner that any rollers except the shielding arrangement holding rollers on the inner side of the pipe are kept away from contact with the pipe during the local shielding of the welded part. In this manner, there remain no scales in the electric resistance welded part, and the toughness of the electric resistance welded part is markedly enhanced.
Patent Literature 3 describes API X80 or higher grade high tensile strength thin-walled electric resistance welded line pipes or tubes. In the technique described in Patent Literature 3, the line pipes are electric resistance welded steel pipes or steel tubes manufactured by subjecting hot rolled steel coils to steps including cold roll forming, electric resistance welding, heat treatment of seam, and sizing treatment, the electric resistance welded steel pipes or steel tubes being such that the outer diameter is 200 mm to 610 mm, the thickness/outer diameter ratio (t/D) is not more than 2%, the metallic structure is an acicular ferrite structure with an average grain size of not more than 5 μm, the oxide-occupied area in the electric resistance welded butt part is not more than 0.1% (1000 ppm), and the tensile strength in a circumferential direction after flattening is not less than 700 N/mm2. These configurations allegedly ensure that the base metal has excellent toughness and the electric resistance welded butt part is as sound as the base metal.
Patent Literature 4 describes electric resistance welded steel pipes or steel tubes for boilers which have few defects in electric resistance welded parts and have excellent creep rupture strength and toughness. The electric resistance welded steel pipes or steel tubes for boilers described in Patent Literature 4 are such that the steel contains, by mass %, C: 0.01 to 0.20%, Si: 0.01 to 1.0%, Mn: 0.10 to 2.0%, Cr: 0.5 to 3.5%, P: not more than 0.030%, S: not more than 0.010%, O: not more than 0.020%, Nb: 0.001 to 0.5%, V: 0.02 to 1.0% and B: 0.0003 to 0.01%, the (Si %)/(Mn %+Cr %) ratio is 0.202 to 1.5, and the area ratio of a ternary oxide mixture of SiO2, MnO and Cr2O3 formed during electric resistance welding is not more than 0.1%. With these configurations, the electric resistance welded steel pipes or steel tubes for boilers allegedly achieve excellent weldability of electric resistance welding and also exhibit excellent creep rupture strength.